1. Field of the Invention
The present invention relates to a sheet treating apparatus detachably or integrally mounted to a main body of an image forming apparatus such as a copying machine or a printer and adapted to treat sheets conveyed from the image forming apparatus main body. In particular, the invention relates to a sheet treating apparatus capable of storing (buffering) sheets conveyed during the treatment of the sheets and an image forming apparatus provided with the sheet treating apparatus, and to a technique of preventing any roller trace and runner trace left on the sheet.
2. Related Background Art
In recent years, a sheet treating apparatus such as a sorter for sorting sheets after image formation is being under development as options on an image forming apparatus such as a copying machine or a laser beam printer, which employs an electrophotographic process. This type of sheet treating apparatus carries out at least one of sort treatment, staple treatment, and alignment treatment on sheets.
In a sheet treating apparatus equipped with a stapler for stapling sheets, the sheets conveyed into a sheet treating apparatus main body are stacked on a treatment tray after passing through a conveyance path inside the main body and then stapled.
The sheet treating apparatus for stapling a sheet bundle stacks the sheets into a bundle on the treatment tray and moves the stapler as stapling means to staple the bundle at one or more positions (generally, two positions). Any sheets for the next job cannot be stacked on the treatment tray during a stapling operation. As a result, a sheet interval must be widened between sheets for each stapling job.
However, widening a sheet interval lowers productivity, in other words, reduces the number of sheets treated per unit time. Proposed as a sheet treating apparatus that solves the drop in productivity is one provided with a sheet holding portion (buffer portion) that holds and keeps the sheets on standby at the midpoint of the conveyance path through which the sheets are conveyed to the treatment tray.
The sheet treating apparatus stores, while treating plural sheets stacked on the treatment tray, plural succeeding sheets in the sheet holding portion and stacks the sheets (buffer sheets) stored in the sheet holding portion on the treatment tray upon completion of the treatment, and then supplies to the treatment tray the remaining succeeding sheets up to the predetermined number.
There are two types of such sheet treating apparatuses provided with the sheet holding portion. First, one type is a rotary buffer type sheet treating apparatus where buffer sheets wind around a roller (see JP H09-048545 A (FIG. 38)). The other type is a horizontal buffering type sheet treating apparatus where buffer sheets are subjected to buffering while kept horizontal on a conveyance path (see JP 2003-081517 A, for example).
The horizontal buffering type sheet treating apparatus requires no roller around which the buffer sheets wind unlike the rotary buffer type one and thus has an advantage in that the apparatus can be reduced in size and cost.
In either the rotary buffer type sheet treating apparatus or the horizontal buffering type sheet treating apparatus, a roller for returning the sheets inside the treatment tray is provided, for example, which keeps rotating relative to the sheet at the same position in pressure contact with the sheet. The treatment tray is not held horizontal, so that the sheet can turn back under its own weight. Accordingly, it is unnecessary to apply a pressure large enough to convey the sheet, thereby leaving no roller trace or runner trace.
However, in contrast to the rotary buffer type sheet treating apparatus, the horizontal buffering type one performs horizontal buffering (horizontally stacks plural sheets on top of one another) on the conveyance path. Hence, the roller capable of conveying the sheets keeps rotating relative to the sheet at rest, at the same position. As a result, a roller trace or runner trace may be left on the buffering sheet.